Typically to rotationally mold a hollow plastic article, a mold that defines the shape of the part to be produced is mounted on the arm of a molding machine. The machine is typically capable of biaxially rotating and moving the mold through the four phases of the process. A predetermined amount of plastic material, in the form of a liquid or a powder, is then placed in the mold's cavity. The machine then simultaneously rotates the mold in two directions and moves the mold into a heating chamber or oven. In the oven, the mold becomes hot and all the plastic material adheres to and sinters onto the inside surface of the cavity. While it continues to rotate, the machine moves the mold out of the heating chamber and into a cooling chamber, where the plastic is cooled to the point that the formed plastic part will retain its shape. The machine then moves the mold to an open station, and the mold stops rotating. The mold can then be opened and the molded part removed. The mold is then recharged with plastic material and the process can be repeated.
In rotational molding, a vent is typically required in communication with various parts of the mold in the form of a tube inserted into the mold to relieve pressure in the mold during the heating cycle and to reduce the negative pressure in the mold during the cooling cycle. Improper venting can result in a blow hole in the form of a void through the molded part due to plugged or improperly sized vents for example.
Due to the many number of vents typically required when molding multiple components, there is increased potential for at least one of the vents to become plugged resulting in blow holes being formed or other potential defects. Inserting the many vents is also time consuming and involves considerable wasted material in the area of each vent.